1. Field of the Invention
The invention relates to an installation in which an operation is performed that requires control over the atmosphere inside a chamber, and relates in particular to an installation for the surface treatment of a running substrate, particularly a polymer film, more precisely for implementing a method whereby the substrate is subjected to an electrical discharge in a gaseous mixture other than air.
2. Description of Related Art
In particular, it is known that the performance of films (surface energy, adhesion properties, etc.), particularly of polyolefin films, can be improved by depositing a small amount of silicon oxide on their surface. To this end, according to these known methods, the substrate (the film) made of polymer is subjected to an electrical discharge with dielectric barrier at a pressure higher than 10 kPa, and, at the same time or subsequently, the substrate thus treated is exposed to an atmosphere consisting of an active gaseous mixture, the active mixture containing, for example, a gas capable of causing the formation of the silicon oxygen deposit, such as a silane (usually a silicon hydrocarbon which may or may not be halogenated, or an alkoxysilane).
In general, treatment by electrical discharge in a gaseous mixture is performed continuously, at substrate running speeds ranging from about ten to several hundred meters per minute, usually in a chamber containing, in addition to the electrodes, a device for injecting the active gaseous mixture and devices for extracting the gaseous emissions. The discharge is created between the electrodes of the chamber arranged on one side of one of the surfaces of the substrate and a counterelectrode arranged on the side of the opposite surface.
By way of illustration of this state of the art of surface treatments of running polymer films, reference may, for example, be made to document EP-0 622 474.
More specifically, the substrate to be treated is applied against the periphery of a metal roll covered with a dielectric material used as a counterelectrode and grounded, along an arc of a circle of several tens of degrees, for example over half the circumference of the roll. A cap delimiting the chamber covers part of the length of this arc of a circle, approximately centrally, extending far enough away from this arc to be able to contain the various elements necessary for the treatment; the length of the roll in its axial direction is slightly greater than the width of the substrate, and the length of the chamber in the same direction is itself generally slightly greater than the width of the substrate.
Furthermore, on an industrial scale, treatment is often performed constantly 24 hours a day, and the apparatus under the cap has to operate as an open system. In spite of the devices for extracting and injecting the active gaseous mixture, the air entrained as a boundary layer at the surface of the strip cannot be completely prevented from getting under the cap. The treatment is therefore carried out in a gaseous mixture consisting of the active gaseous mixture injected and of air, the proportions of which are poorly controlled.
The active gaseous mixtures employed are generally binary or ternary mixtures consisting of nitrogen by way of carrier gas and of active gases in proportions of the order of a few hundreds of ppm. Work carried out in optimizing the method clearly demonstrates that the level of each of the active gases in the mixture is an essential parameter that needs to be controlled as strictly as possible. Furthermore, certain active gases such as silanes have the property of reacting strongly with air to give byprnducts, particularly solid and gaseous ones.
It is therefore necessary to have use of a treatment installation capable of good control over the treatment gaseous atmosphere, by reducing the amount of incoming air as far as possible.
There are also known (see, for example, the European patent mentioned earlier), installations for implementing such methods comprising injectors for injecting active gaseous mixture right inside the discharge and extraction slits at the inlet and outlet of the chamber. The injection and extraction flow rates are regulated so that the amount of incoming air is minimized. However, minimal air content achieved under the cap remains high by comparison with the content of active gas in the injected mixture. Hence, when the desired treatment level demands that the gaseous mixture contains a very low content of oxidizing gas, uncontrolled addition of a certain amount of air (and therefore oxygen, which is an oxidizing gas) to the gaseous mixture leads to poor control of the quality of the treatment. Control over the composition of the active gaseous mixture may therefore prove insufficient with this type of installation.
The object of the invention is to overcome this drawback and more particularly to provide an improved installation of the aforementioned type (in which an operation is performed that requires control over the atmosphere inside a chamber), and, in the particular case of the surface treatments of running substrates, to propose an installation such that the control and management of the gas are possible both in terms of incoming gases and exiting gases, through the fact that:
the air entrained on the surface of the substrate is prevented from entering the treatment chamber;
the active gaseous mixture is injected into the chamber and thus used optimally for the actual treatment;
any gaseous emissions are collected so that they can be processed before they are rejected to the air.
To this end, the invention relates first of all to an installation in which an operation is performed that requires control over the atmosphere inside a chamber, the operation being performed in the presence of a gaseous mixture capable of giving off emissions, characterized in that it comprises:
inlet and outlet devices adjoining the chamber to oppose respectively the ingress of air into the chamber and the exit of gaseous emissions therefrom;
an extraction device comprising a duct opening into the chamber; and
means for regulating the flow rate of gas drawn out by said extraction device so as to maintain an approximately zero pressure difference between the inside of the chamber and the surrounding atmosphere.
The expression xe2x80x9capproximately zeroxe2x80x9d according to the present invention is to be understood as meaning a pressure difference (positive or negative) not exceeding a few tens of Pa or even 100 Pa. However, it is preferable according to the present invention to work with pressure differences not exceeding 50 Pa or even differences lower than 10 Pa.
The installation according to the invention may also have one or more of the following features:
the inlet device comprises means for injecting an inert gas forming a gas knife upstream of the chamber, means for creating a raised pressure discharging the inert gas away from the chamber forming a piston preventing air from entering the chamber and means for ducting the air away from the chamber.
the outlet device comprises means for injecting an inert gas forming a gas knife downstream of the chamber, means for creating a raised pressure discharging the inert gas toward the chamber forming a piston opposing the entrainment of emissions out of the chamber, and means for ducting toward the chamber any emissions that may be trying to leave this chamber.
said means for injecting the inert gas forming a gas knife comprise a flat-walled gas-injection slit opening to inside the inlet or outlet device concerned.
said means for creating a raised pressure discharging the gas and forming a piston comprise open grooves facing the interior space of the inlet or outlet device concerned and forming a labyrinth.
said ducting means comprise a passage separated from said gas-injection means by a partition and open facing the interior space of the inlet or outlet device concerned.
said ducting means comprise a passage cut into one face of a plate separated from another part by a space forming an inert-gas-injection slit.
the ratio between the length and the height of the passage is at least equal to 3, preferably at least equal to 6.
the installation further comprises lateral devices outside the chamber, comprising at least means for creating a raised pressure discharging the gas away from the chamber and forming a piston preventing air from entering the chamber and, as appropriate, also means for injecting an inert gas forming a gas knife to the side of the chamber, and means for ducting the air away from the chamber.
the installation comprises a second extraction device downstream of the outlet device.
it comprises a controller for coordinating the operation of all or some of the elements of the installation.
the controller is capable of optimizing the flow rates of injected gas (whether this is the inert gas or the gaseous mixture used for the actual operation), according to oxygen content readings for evaluating the air intakes taken at various points inside the chamber and/or according to a reading of the differential pressure between the inside of the chamber and the surrounding atmosphere.
the operation performed in the chamber is an operation of surface-treating a running substrate by an electrical discharge in the presence of said gaseous mixture capable of giving off emissions.
the operation performed in the chamber is an operation of crosslinking a coating (for example an ink or a varnish) by ultraviolet radiation (known as UV curing) or using an electron beam, in the presence of an inert, for example nitrogen-based gaseous mixture.
The idea of a xe2x80x9cgas knifexe2x80x9d according to the invention is to be understood as covering, in general, varied gas-injection geometries and therefore not necessarily injections of very narrow shape and extremely high gas speed (as is sometimes to be understood by the expression xe2x80x9cknifexe2x80x9d in certain literature).
All the features and advantages of the invention will be better understood by examining one of its practical applications, with the case of installations for the surface treatment of a running substrate by an electrical discharge in the presence of a gaseous mixture capable of giving off emissions.
The present invention therefore also relates to an installation for surface-treating a running substrate by an electrical discharge in the presence of a gaseous mixture capable of giving off emissions, comprising a substrate support against a region of which the substrate is applied, and electrodes near the support which forms a counterelectrode, characterized in that it also comprises:
a chamber adjacent to the support over at least the width of the substrate and along at least part of said region, in which the electrodes are housed;
inlet and outlet devices adjoining the chamber to oppose respectively the ingress of air into the chamber and the exit of gaseous emissions therefrom;
an extraction device comprising a duct opening into the chamber; and
means for regulating the flow rate of gas drawn out by said extraction device so as to maintain an approximately zero pressure difference between the inside of the chamber and the surrounding atmosphere.
By virtue of this design, the chamber is sufficiently isolated from the outside, while at the same time maintaining an open-system operation, so that the installation is capable of treating a substrate running at high speed.
The surface treatment installation according to the invention may further exhibit one or more of the following features:
the installation comprises a gaseous mixture supply manifold connected to the electrodes to inject the gaseous mixture against the substrate amongst the electrodes.
the inlet device comprises means for injecting an inert gas against the substrate forming a gas knife upstream of the chamber, means for creating a raised pressure discharging the inert gas away from the chamber forming a piston preventing air from entering the chamber and means for ducting the air away from the chamber.
the outlet device comprises means for injecting an inert gas against the substrate forming a gas knife downstream of the chamber, means for creating a raised pressure discharging the inert gas toward the chamber forming a piston opposing the entrainment of emissions by the substrate, and means for ducting toward the chamber any emissions that may be trying to leave this chamber.
it further comprises lateral devices outside the chamber, comprising at least means for creating a raised pressure discharging the gas away from the chamber and forming a piston preventing air from entering the chamber and, as appropriate, also means for injecting an inert gas against the support forming a gas knife to the side of the chamber, and means for ducting the air away from the chamber.
said means for injecting the inert gas forming a gas knife comprise a flat-walled gas-injection slit opening to the outside opposite the support (the slit advantageously is a few tenths of a millimeter thick).
said means for creating a raised pressure discharging the gas and forming a piston comprise open grooves facing the support and forming a labyrinth.
said ducting means comprise a passage separated by a partition from said inert-gas injection means and open facing the support.
said ducting means comprise a passage cut into one face of a plate separated from another part by a space forming an inert-gas-injection slit.
the ratio between the length (in the direction of travel of the substrate) and the height of the passage is at least equal to 3, preferably at least equal to 6.
the substrate support is a rotary roll and the region against which the substrate is applied is a region in the form of an arc of a circle.
the means for regulating the drawn-out flow rate comprise a regulating valve in the extraction duct opening into the chamber, and a control circuit comprising a pressure sensor for controlling the valve.
the means for regulating the drawn-out flow rate comprise means for regulating the rotational speed of the extraction fan (for example, of the frequency variator or potentiometer type) and a control circuit comprising a pressure sensor for controlling the rotational speed of the extraction fan.
it comprises a controller for coordinating the operation of all or some of the elements of the installation.
the controller is capable of optimizing the flow rates of injected gas (whether this be the inert gas or the gaseous mixture used for the actual surface treatment), according to oxygen content readings for evaluating the air inlets taken at various points inside the chamber and/or according to a reading of the differential pressure between the inside of the chamber and the surrounding atmosphere.
it comprises a second extraction device downstream of the outlet device.
the electrodes are connected to a so-called high-voltage and high-frequency power supply.